The recent emergence of the pandemic swine-origin 2009 A(H1N1) influenza virus strongly emphasises the potential of influenza viruses to cause morbidity and mortality in the human population on a global scale. Worldwide over 200 countries and overseas territories or communities have reported laboratory-confirmed cases of the pandemic virus including more than 16,000 deaths [1]. Vaccination is the primary method to prevent or lower the burden of influenza disease. However, as illustrated again by the 2009 pandemic, a rapid response during the early phase of an outbreak is hampered by the time-consuming vaccine strain preparation and vaccine manufacturing process currently used. This, combined with the notorious capacity of influenza viruses to escape from existing immunity by antigenic drift and shift, stresses the need for novel, safe and preferably broadly effective vaccines that can be produced rapidly and in flexible response to newly emerging antigenic variants.
The currently licensed influenza virus vaccines are composed of the viral envelope glycoproteins, the hemagglutinin (HA) and neuraminidase (NA). Antibodies elicited by these two large glycoproteins have distinct properties in immunity against influenza virus. Antibodies to HA generally neutralise viral infectivity by interference with virus binding to sialic acid receptors on the target cells or, subsequently, by preventing the fusion of the viral and cellular membrane through which the viral genome gains access to the target cell. Antibodies to NA disable release of progeny virus from infected cells by inhibiting the NA-associated receptor destroying enzymatic activity. The HA-mediated humoral immunity has been characterised most extensively and shown to prevent virus infection. The contribution of NA antibodies to preventing disease has been studied less well. They appeared to produce a kind of permissive immunity [2] characterised by a decrease in infectious virus release from apical surfaces of infected epithelia [2-8], reducing the probability of virus shedding and spread into the environment.
Immunisation with the combination of HA and NA provides enhanced protection against influenza [5, 9, 10]. Although HA and NA are equivalently immunogenic [2], the humoral immune response towards conventional inactivated vaccines or virus infection is naturally skewed towards HA since HA and NA occur on the viral surface at an approximately 4:1 ratio [11]. In addition, in intact virions the HA immunologically outcompetes NA in B and T cell priming as shown in mice [12]. This antigenic competition is not seen in vaccinated animals when HA and NA are administered separately [10, 13]. The currently licensed pandemic vaccines as well as the seasonal trivalent vaccines are generally prepared from whole viruses and are hence biased to contain more HA than NA antigen. Adapting the HA-NA ratio in vaccine formulations in favour of NA may provide a more balanced humoral immune response resulting in higher NA antibody levels and increased protection against disease [3, 14].
Since the current inactivated influenza virus vaccines are standardised only for the amount of HA, the NA content is variable as is, consequently, the frequency and level of seroconversion to NA, which is often rather poor [28, 29].
Typical for influenza A viruses, antigenic variants of HA and NA within a certain virus subtype able to escape from existing immunity are gradually selected in the human population. This process of antigenic drift calls for the almost annual adjustment of the seasonal vaccine composition in response to newly arising variants. In view of the threat of future influenza pandemics, caused for instance by an avian H5N1 virus, there is a need for vaccines inducing broadly protective immunity.